Indirect
calorimetry is the most accurate measurement of energy generated 
and calories burned  by your body during rest and muscular effort.
Unfortunately, it is an impractical, invasive procedure which requires:

A
fully enclosed laboratory setting where normal daily activities are
impossible to replicate

A
mouthpiece for breathing

Computerized
systems for the collection, analysis, and output of respiratory data

LifeChek
uses direct calorimetry and is the only portable device which achieves
results equivalent to indirect calorimetry.